Factors affecting the encapsulation of drugs within liposomes

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Abstract
The reported efficiencies of drug encapsulation into liposomes range from < 0.1% to > 10%/.mu.mol phospholipid depending on the nature of the drug and of the liposome employed. Factors which control the efficiency of drug encapsulation were studied. Most polar drugs are sequestered within the internal aqueous compartment of the liposomes; nonpolar drugs can bind to the liposome membrane in addition to being sequestered, accounting for their higher encapsulation efficiencies. The encapsulation of nonpolar drugs, but not of polar drugs, is very sensitive to the physical characteristics of the liposome membrane; in particular a fluid membrane favors the efficient encapsulation of nonpolar compounds. The drug cytosine arabinoside is anomalous in that this highly polar compound seems to interact with the liposome membrane at physiological conditions of pH and ionic strength, allowing it to be encapsulated with high efficiency.